****Neutron Protection

Question 1

What are the sources of neutrons?

Answer 1

Radionuclides, accelerators (produced by ion beams or photonuclear reactions), nuclear reactions (prompt fission nuetrons, delayed neutrons, photoneutrons, gamma radiation), and nuclear fuel reprocessing plants.

Question 2

Describe the properties of Beryllium.

Answer 2

Large nucleus; small atom
Close packed crystal structure
Can be formed into high purity metal components
Almost transparent to x-rays
High cross section for high energy alphas or protons
Neutrons discovered by irradiating Be with alphas
e.g. Be-9 + He-4 = C-12 + n-1
Be-9 binding energy per neucleon: 6.46 MeV

Question 3

What is the binding energy per nucleon of Beryllium, and why does this mean it is useful for neutron production?

Answer 3

6.46 MeV
Beryllium is used as has a low binding energy, so is easy to release neutrons from nucleus as less energy required to overcome binding energy. So high energy alphas or protons produce high energy neutrons.

Question 4

Describe the main principles of neutron therapy.

Answer 4

Cyclotron-produced protons (26-66 MeV) onto Be target (more common).
Deuterium (12.5-14 MeV) onto Tritium target (less common).
Therapeutic beams are mixture of x-rays and neutrons

Question 5

What is photodisintegration?

Answer 5

A photon passes through an electron cloud and interacts with a nucleus. This produces a neutron (photoneutron) or possible a proton or alpha particle, and an ionising recoil nucleus with a changed mass number and/or atomic number.

I.e: Photon hits a nucleus, if energy>B.E = recoil nucleus (with changed mass/atomic number) + photoneutron (or maybe proton/alpha).

Question 6

What is the threshold photon energy to produce a photo-neutron?

Answer 6

10 MeV

Question 7

What is the threshold photon energy to produce an alpha or proton from photodisintegration?

Answer 7

Much higher than 10 MeV.

Question 8

What is the mass energy conservation equation?

Answer 8

hv + m(0)c^2 = m(1)c^2 + k(1) + m(n)c^2 + k(n)
where:
hv is energy of incoming LINAC photon
m0c2 is the initial mass-related energy of the nucleus
m1c2 is the final mass-related energy of the nucleus
mnc2 is the final mass-related energy of the neutron
k1 and kn are final kinetic energies of nucleus and neutron

Question 9

What are the consequences of neutrons in a linac bunker?

Answer 9

Contribute to patient dose
Contribute to a radiation hazard at the maze
Present an additional radiation hazard at the treatment as a result of activated products.

Question 10

What are the 4 types of neutrons and at what energy ranges do they occur?

Answer 10

Thermal neutrons (<0.4 keV)
Intermediate neutrons (0.4-200 keV)
Fast neutrons (200 keV - 10 MeV)
Relativistic neutrons (>10 MeV)

Question 11

Describe thermal neutrons.

Answer 11

Energy distribution same as atoms and molecules of surrounding medium
Maxwellian distribution of velocities
(Epithermal neutrons: 0.4 eV – 100 eV)
Most common interaction with matter is neutron capture but other reactions may also occur

Question 12

Describe intermediate neutrons.

Answer 12

Produced as result of elastic scattering of fast neutrons in materials with low atomic number e.g. carbon or hydrogen or in the human body
Interact with matter by elastic scattering

Question 13

Describe fast neutrons.

Answer 13

Interact with light nuclei primarily through elastic scattering
Although inelastic scattering predominates with higher energies
fast-neutron monitoring instruments become insensitive and inaccurate below 200 keV

Question 14

Describe relativistic neutrons.

Answer 14

Inelastic scattering more important than elastic
elastic cross-section negligible for high atomic number materials
main form of non-elastic collision ejection of protons or neutrons from target nucleus
At very high energies energy appearing as gamma radiation negligible c.f. energy transferred to cascade protons, neutrons and other nuclei.

Question 15

What are the 5 interactions of neutrons with matter?

Answer 15

Elastic scattering: (n,n)
Inelastic scattering: (n,n'), (n,nγ)
Capture: (n,γ)
Non-elastic reactions: (n,n), (n,p), (n,d), (n,α), (n,t), (n,αp), etc.
Fission: (n,f)

Question 16

Describe elastic scattering for neutrons.

Answer 16

Neutron shares initial kinetic energy with the target nucleus
Target nucleus suffers only recoil and is not left in an excited state
Total kinetic energy in system remains constant.
Momentum is conserved.

Question 17

Describe inelastic scattering for neutrons.

Answer 17

only possible with fast neutrons
Total energy of scattered neutron and recoil nucleus < incident neutron
nucleus left in excited state
In (n,nγ) process, excitation energy released by nucleus via emission of prompt gamma ray
In (n,n’) process, nucleus remains in a metastable state

Question 18

Describe neutron capture.

Answer 18

Thermal neutron capture possible in nearly all nuclides
Target nucleus captures incident neutron
Forms compound nucleus left in excited state
Excitation energy may be emitted in one or more gamma rays
Some elements have a high cross-section (proportional to 1/v) for thermal neutron capture
Others (e.g. gold) show resonance capture

Some detectors make use of this – measures the gamma emissions from the material that is activated by the neutrons & infers what neutron flux was to create those emissions.

Question 19

Describe neutron non-elastic reactions.

Answer 19

Incident neutron captured by the target nucleus
particles (protons, deuterons, alpha particles, tritons, etc.) emitted
(n, 2n) reaction can occur at incident energies above 10 MeV
These reactions are not usually uniform with incident energy but show resonances which make calculations of the interactions of a complex neutron spectrum difficult

Question 20

Describe neutron fission.

Answer 20

Interactions with fissile nuclei cause formation of a compound nucleus
compound nucleus then splits into two fission fragments and one or more neutrons.
May occur in isotopes of Th, U, Np, Pu and higher actinides
Can use these materials as neutron detectors because relatively simple to detect the fission products
Occurs at practically all neutron energies but cross-section for fission considerably higher for 233U, 235U and 239Pu when thermal neutrons incident
In 232Th and 238U practically no fissions take place at neutron energies < 1MeV.

Question 21

How does RBE change with neutron energy?

Answer 21

As energy increases, RBE increases.

Question 22

Which personal dosimeters can be used for neutron dosimetry?

Answer 22

Thermoluminescent albedo dosimeters
Electrochemically etched plastics (CR-39)
Bubble dosimeters

Question 23

Describe a thermoluminescent albedo dosimeter.

Answer 23

Albedo is to do with reflection rather than incident measurement of neutrons. Neutrons entering human body are moderated & back-scattered which creates neutron fluence at body surface. (especially in thermal- and intermediate-energy ranges)

LiF TLD chip designed to detect thermal neutrons

Question 24

Describe electrochemically etched plastics.

Answer 24

Nuclear track detector - widely used
Neutrons detected by path of damaged molecules in material where the tracks are detected by suitable etching process: chemical etching (CE), electrochemical etching (ECE), or both combined.
Calibration of track density / neutron dose equivalent required.
(Etches away anything that hasn’t been damaged = only left with tracks from neutrons = can count number of tracks. )

Question 25

What are the disadvantages of a thermoluminescent albedo dosimeter?

Answer 25

Disadvantages:

• relatively low response in fast neutron energy range
• high contribution of incident thermal neutrons on dose indication

Question 26

What are the advantages of a thermoluminescent albedo dosimeter?

Answer 26

Advantages:

• no energy threshold for detection
• extended dose range from 0.1 mSv to about 10 Sv
• low fading for longer monitoring periods
• small influence of body size on dosimeter reading
• low dependence on direction of incident radiation if at least two dosimeters have been worn on the front and on the rear of the body
• acceptable gamma dose discrimination

Question 27

What are the disadvantages of electrochemically etched plastics?

Answer 27

Disadvantages:
-Batch variations due to lack of dosimetry grade material
-Significant angular dependence
-Sensitivity
-Under-responds for certain neutron energy ranges
lower energy neutrons from reactors or high energy accelerator-produced neutrons

Question 28

What are the advantages of electrochemically etched plastics?

Answer 28

Advantages:

• Fast neutron effective response
• Low neutron energy threshold
• Photon insensitivity

Question 29

Describe a neutron bubble detector.

Answer 29

A small container (measuring several cm.) filled with superheated Freon droplets within an elastic clear polymer.
Recoil protons are produced by neutron interactions with the polymer.
Protons striking Freon droplet may vaporize it, which remains trapped as a visible bubble in the polymer.
Count number of bubbles to determine neutron fluence.
Recharging is accomplished by pressuring the polymer container above vapor pressure of Freon gas mixture
reforms bubbles to liquid droplets = all becomes clear again.

Question 30

What are the advantages of a bubble detector?

Answer 30

Advantages:

• very sensitive
• detection limit of few microSv
• neutron energy threshold of standard option 100 keV
• need no electronics or power to operate
• cannot be disturbed by electromagnetic interference
• insensitive to photons

Question 31

What are the disadvantages of a bubble detector?

Answer 31

Disadvantages:
-strong temperature dependence
-temperature correction can be used to minimise effect
severe physical shock sensitivity – impact number of visible bubbles
-sharp impact causes superheated drops to vaporise & form bubbles like neutron- induced ones
-cannot be used to accumulate dose information for longer than few days: over time, bubbles spontaneously reform into liquid.

Question 32

What is the neutron attenuation coefficient equal to?

Answer 32

Attenuation coefficient =

Atoms per unit volume x cross section

Question 33

Does hydrogen have a large or small neutron cross section?

Answer 33

As neutrons only interested in nucleus – smaller cross section = more interaction.
Hydrogen – large cross section as nucleus in comparison to atom is much bigger.

Question 34

Describe the principles of a boron tri-fluoride detector.

Answer 34

A neutron interacts with boron which produces Li-7 plus an alpha particle. Both are highly charged and can be collected and counted.
Elemental boron is not a gas but BF3 and doubles as a quenching media in the detector.
The detector voltage is set in the proportional range to enable lower gamma peaks to be discounted.

Area monitors have a single axial BF3 detector surrounded by large polythene to moderate most energetic neutrons to thermal energies (this makes up the bulk of the detector). A perforated boron or cadmium insert is used to improve energy response. Approx 2500V is within the proportional range.
Modern versions have He-3 fill instead to comply with air carriage regulations.

Question 35

What are the specifications of a boron tri-fluoride detector?

Answer 35

Energy – around +/-10% of ICRP
Range 1-100,000 µSv per hour
Linearity – within a few percent
Gamma (X-ray) rejection about 10-6
Not good enough for main beam LINAC use
Lacks spectral data for accurate dosimetry
Only gives single figure for neutron energy

Question 36

Describe Bonner spheres.

Answer 36

Series of polythene moderating spheres - each one a different size.
Central interchangeable detector (passive or active) - either TLD, Li-6 scintillation detector, or Gold foils.
Each sphere uniquely modifies the neutron spectrum - so can quantify the neutron spectrum.
Residual thermal neutron signal at centre.

The data is ‘unfolded’: the detector signal = sum of nuetron flux in specific band x response of sphere in same band.

Question 37

Describe the possible detectors used at the centre of a Bonner sphere.

Answer 37

Thermo-luminescent dosimeters:
Li6F and Li7F have different thermal neutron sensitivities but similar gamma/x-ray sensitivities so together reasonable gamma rejection; limited accuracy

Lithium6 iodide scintillation detectors:
Connected to PM tube via light pipe; can energy discriminate gammas from thermal neutrons but cannot be used with LINACS or other RF equipment

Gold foils Au197(n,γ) Au198:
Activation counted (T½ = 2.7 days); neutron specific so excellent gamma rejection and reproducible

Question 38

What is the typical neutron dose at the patient plane?

Answer 38

Range 10-6 Sv per Monitor Unit at 15MV; 2.6x10-5 at 20MV

Question 39

What could increase the neutron dose to the patient?

Answer 39

High neutron back-scatter if metal shielding used in primary barrier (higher for lead than steel than concrete).

Question 40

Up to which energy is normal concrete and steel barriers appropriate for neutron shielding?

Answer 40

15 MV

Question 41

What is a good wall lining agent against neutrons, and why?

Answer 41

Lithium salts, as it has a high hydrogen content.

Question 42

Why is it necessary to leave a gap after exposures (approx 30 minutes) before working on a Linac head?

Answer 42

Neutron activation may be present. Especially at or above 10MV.
Dose rates immediately after beam off can be 10’s of µSv/hr.

Question 43

Other than significant doses, what can neutron dose also cause?

Answer 43

Electronic failure of equipment. Mechanism may be intensely ionising recoil ions.